1. Technical Field
The present invention relates to an image forming apparatus, in which a toner image formed on a transfer member, based on image data, is transferred and fused on a recording medium to form an image.
2. Description of the Background Art
In electrophotographic image forming apparatuses, toner images are formed on a transfer belt using image data, and then transferred and fused on a sheet to form images. Such image forming apparatuses can form images using colored toner such as yellow, magenta, cyan, and black toner, and also clear toner. For example, an image forming apparatus can form images having a watermark on the top layer of the images by superimposing a toner image of clear toner over the toner images of colored toner, and transferring and fusing each of toner images on a sheet.
Compared to a color image formation using only the colored toner, when an image is formed using the colored toner and the clear toner, the total amount of toner to form the image becomes great. However, in image forming apparatuses using electrophotography, if the total amount of toner formed on the transfer belt using the becomes great, a transfer bias set for the normal transfer process may not be enough for transferring the toner image from the transfer belt to a sheet, and resultantly a transfer failure may occur.
In light of such problem, JP-2009-63744-A discloses an image forming method of an image forming apparatus, in which an upper limit (or control value) is set for the total amount of toner of colored toner and clear toner. When the total amount of toner exceeds the upper limit, the density or concentration of colored toner is adjusted. In such a method, when the total amount of toner exceeds the upper limit, the density or concentration of clear toner is fixed to a given value, and the density or concentration of colored toner is decreased to limit the total amount of toner at the upper limit. With such a configuration, an image can be formed without changing gloss appearance, which is an effect of the clear toner. However, such toner-amount reduction of the colored toner undesirably decreases image density of the resultant output image.